Natural Kinds and Concepts: A Pragmatist and Methodologically Naturalistic Account Ingo Brigandt Abstract: In this chapter I lay out a notion of philosophical naturalism that aligns with prag- matism. It is developed and illustrated by a presentation of my views on natural kinds and my theory of concepts. Both accounts reflect a methodological naturalism and are defended not by way of metaphysical considerations, but in terms of their philosophical fruitfulness. A core theme is that the epistemic interests of scientists have to be taken into account by any natural- istic philosophy of science in general, and any account of natural kinds and scientific concepts in particular. I conclude with general methodological remarks on how to develop and defend philosophical notions without using intuitions. The central aim of this essay is to put forward a notion of naturalism that broadly aligns with pragmatism. I do so by outlining my views on natural kinds and my account of concepts, which I have defended in recent publications (Brigandt, 2009, 2010b). Philosophical accounts of both natural kinds and con- cepts are usually taken to be metaphysical endeavours, which attempt to develop a theory of the nature of natural kinds (as objectively existing entities of the world) or of the nature of concepts (as objectively existing mental entities). However, I shall argue that any account of natural kinds or concepts must an- swer to epistemological questions as well and will offer a simultaneously prag- matist and naturalistic defence of my views on natural kinds and concepts. Many philosophers conceive of naturalism as a primarily metaphysical doc- trine, such as a commitment to a physicalist ontology or the idea that humans and their intellectual and moral capacities are a part of nature. Sometimes such legitimate views motivate a more contentious philosophical program that main- tains that any philosophical notion ought to be defined in a purely physicalist vocabulary (e.g., by putting forward a theory of concepts and intentional states that does not define them in terms of intentional notions). We will see that I re- ject this latter project (which is naturalistic in some sense) on naturalistic grounds, as science does not aim at developing reductive definitions. Rather than naturalism as a metaphysical doctrine, more germane to my account is a methodological type of naturalism. Here the idea is that some aspects of scien- tific method and practice should be used by philosophers in their attempts to de- velop philosophical accounts. I will illustrate this naturalistic method by laying out how philosophers can and ought to develop philosophical notions without simply relying on their personal intuitions or folk intuitions as revealed by ex- perimental philosophy surveys. The starting point of this method is that philoso- 172 Ingo Brigandt phical concepts are introduced for specific philosophical purposes – just like scientific concepts are used for particular epistemic purposes – and that these purposes determine the appropriateness of a philosophical analysis. For exam- ple, I will defend my account of what a ‘concept’ is in terms of its fruitfulness for explaining phenomena of interest to philosophers. My methodological natu- ralism aligns with a pragmatist perspective, since science is to a large extent a pragmatic enterprise and since scientists freely invoke scientific values and in- terests and defend their accounts in terms of their fruitfulness at meeting scien- tific aims. Natural kinds The traditional philosophical aim of an account of natural kinds is to provide a metaphysical characterisation of what a natural kind is, which among other things distinguishes natural kinds from other kinds and entities (Bird and Tobin, 2009). Given that, broadly speaking, a natural kind is a grouping of objects that corresponds to the objective structure of nature, an account of natural kinds must explain how natural kinds differ from nominal kinds, i.e., a grouping of various objects that is merely the result of human convention. One possibility is to de- fine natural kinds as those kinds that figure in laws of nature (Fodor, 1974). An- other, though not necessarily conflicting account may construe a natural kind as characterised by an essence, i.e., some intrinsic, structural property that all kind members share and that causes the typical properties associated with the kind (Putnam, 1975). For instance, the essence of oxygen is its atomic structure, in- cluding the number of subatomic particles making up an oxygen atom. This atomic structure explains in which chemical reactions oxygen can participate, and other chemical properties characteristic of the natural kind oxygen. Functional kinds are generally considered to not be natural kinds; for if kind membership is defined by members having a common function, such a kind is bound to be structurally heterogeneous. This is because any one function is mul- tiply realizable, i.e., there are different actual or possible physical entities that realize this function in one way or another. For instance, instances of ‘money’ as a functional kind include different metals (gold, coins), different objects made of paper (bills, cheques), and electronic states and information (computerized bank accounts). A functional kind from ecology such as ‘predator’ is likewise multi- ply realized. There is a plethora of predatory species across the animal kingdom that differ substantially in their structural-anatomical, physiological, develop- mental, and even behavioural properties. The structural heterogeneity of func- tional kinds is usually considered to be incompatible with them being natural kinds, which are deemed to be characterized by a shared structural essence. Natural Kinds and Concepts 173 Yet a look at biology suggests that many natural kinds are heterogeneous. A case in point is species and higher taxa, which have been taken to be prime ex- amples of natural kinds in biology. The different individuals forming a biologi- cal species (e.g., orangutans) or a higher taxon (e.g., vertebrates) can be very different from each others. This variation is a biological reality and of scientific importance (e.g., underwriting the ability of species to evolve), so heterogeneity need not be an accidental feature, but can be constitutive of some kinds (Wilson et al., 2007). A good metaphysical solution is to make use of Richard Boyd’s (1991, 1999a, 1999b) suggestion that natural kinds are homeostatic property clusters (HPC kinds). The identity of an HPC kind is, in general, not determined by a single essential property; instead, there is a cluster of properties that are correlated. Most of the kind members possess most of these properties, but none of the properties in the cluster has to be shared by all kind members, permitting variation among the members of an HPC natural kind. A requirement is that the correlation of properties is not an accident, but due to some mechanisms that causally maintain the correlation (thus the label ‘homeostatic’ property cluster). Thereby our grouping of objects into a kind based on such a cluster of correlated properties conforms to features in nature and HPC kinds are indeed natural rather than nominal kinds (Wilson et al., 2007). Given the possibility of heterogeneous natural kinds, one may wonder whether at least some functional kinds are natural kinds after all. One option is to try to assess how many properties are correlated in a given kind, and count those kinds as natural where a sufficient number of properties are correlated. However, this approach ultimately results in a continuum between nominal kinds and natural kinds. More importantly, simply counting the number of prop- erties correlated (or assessing the degree of homogeneity) fails to pay attention to the crucial epistemic role that natural kinds have for science. Natural kinds are important because they permit induction and explanation. In his discussion of the problem of induction, Nelson Goodman (1955) prominently argued that reli- able induction requires projectible predicates. Within a contemporary, realist framework (which I adopt), it is clear that a predicate referring to a natural kind is projectible. The reliable correlation of properties in an HPC kind, for instance, grounds induction and other instances of scientific inference. Many natural kinds support scientific explanations, e.g., if the kind figures in laws or if the kind’s essence (or one of the properties from the cluster defining an HPC kind) causes some of the features typically associated with a kind. As I have argued in a previous paper, illustrated by biological examples (Brigandt, 2009), the above considerations suggest that the main philosophical task is not to offer a metaphysical construal of what a natural kind is, but an epistemological study of (i) what inferential and explanatory aims scientists pur- sue with the study of a certain natural kind, and (ii) how well a grouping of ob- 174 Ingo Brigandt jects into a kind meets such inferential and explanatory aims.1 For also many functional kinds studied for instance in ecology, physiology, psychology, or economics figure in scientifically important generalizations and explanations. While ‘money’ is multiply realized, it is part of many macroeconomic generali- zations, such as Gresham’s law. Ecological generalizations exist for the tempo- ral change of the sizes of predator and prey populations. Such generalizations do not describe internal, structural aspects of the members of a functional